F2 Laser Research Articles

Photochemical surface modification of amorphous carbon thin film to transparency in atmosphere of excited singlet oxygen atoms O(1D) under 157 nm F2 laser irradiation

Amorphous carbon (a-C) thin films have various useful properties; thus, their applications are wide-ranging. These properties are characterized by the ratio of sp3 to sp2 carbon in the thin film, and many studies have been reported on changing this ratio. In this paper, we used an F2 laser with a wavelength of 157 nm to photoexcite the surface of an a-C thin film, and simultaneously photodecompose atmospheric oxygen molecules O2 into excited singlet oxygen atoms O(1D). The results showed that O(1D) atoms can photochemically etch sp2 carbon in thin films significantly better than triplet oxygen atoms O(3P). Thus, we found that the surface layer of the a-C thin film was photochemically modified to be transparent. Raman spectroscopy revealed no change in the chemical bonding state of the transparently modified a-C thin film. On the other hand, in X-ray photoelectron spectroscopy, the intensity of the C 1s peak originated to sp2 carbon decreased. The refractive index was also slightly lowered, probably due to lowering film density. In addition, droplets mainly composed of sp2 carbon generated during thin film deposition were significantly removed after F2 laser irradiation. The electrical resistivity slightly decreased upon F2 laser irradiation and remarkably decreased as the substrate was heated, exhibiting low bandgap properties. Therefore, we succeeded in discovering a new method for changing the ratio of sp3 to sp2 carbon in a-C to be transparent thin film by preferentially generating O(1D) atoms using the F2 laser, as a new elemental technology for fabricating transparent flexible electronic devices.

F2 Laser-Induced Micro-Reticulated Structural Changes of Amorphous Carbon Thin Films

Micro/nanoprocessing of materials using lasers is currently an active research topic. In that research, the choice of the laser to be used is critical, but the F2 laser, which has the shortest wavelength (157 nm) among commercially available lasers, has few research compared to its potential. In this paper, we discovered a new photochemical processing by using an F2 laser to irradiate an amorphous carbon thin film. The short wavelength and high photon energy of the F2 laser can photoexcite the surface of the thin film at high density and generate active oxygen atoms O(1D) by photodecomposition of atmospheric oxygen molecules. As a result, the optical change of the amorphous carbon thin film was induced without thickness reduction, and a micron-sized network-like, reticulated structural change was formed in the thin film surface after one month at the latest. The formed micron-sized reticulated structure was relatively swollen, and a graphitization occurred in the structure, observed by Raman spectroscopy. However, the structure was not observed when the laser irradiated area became smaller. This work has made it possible to form a micron-sized reticulated structure including carbon nanocrystals in an amorphous carbon, which is expected to further expand the applications of carbon materials.

Mid-infrared laser operation of Er3+-doped BaF2 and (Sr,Ba)F2 crystals.

We report on the first, to our knowledge, mid-infrared laser operation of two Er3+-doped barium-containing fluorite-type crystals, BaF2 and (Sr,Ba)F2, featuring a low-phonon energy behavior. A continuous wave 4.9 at.% Er:(Sr,Ba)F2 laser generated 519 mW at 2.79 µm with a slope efficiency of 25.0% and a laser threshold of 27 mW. The vibronic and spectroscopic properties of these crystals are determined. The phonon energy of (Sr,Ba)F2 is as low as 267 cm-1. The Er3+ ions in this crystal feature a broadband emission owing to the 4I11/2 → 4I13/2 transition and a long luminescence lifetime of the 4I11/2 level (10.6 ms) making this compound promising for low-threshold, broadly tunable, and pulsed 2.8-µm lasers.

The patterned Au oxide layer formation on Au surfaces by F2 laser irradiation under the atmospheric conditions

The patterned Au oxide layer on the Au plate surface was produced by F2 laser irradiation under atmospheric conditions. The Au oxide formation after F2 laser irradiation was confirmed by X-ray photoelectron spectroscopy with the formation of a pair of peaks of Au 4f at 85.4 and 89.0 eV. The stability of Au oxide in air, vacuum, ethanol, H2O, and hexane was examined. The Au oxide in the vacuum condition shows a much longer lifetime than in air, and hexane shows a long lifetime comparable in air. As a possible application of the Au oxide pattern on Au, we demonstrated the patterning of the self-assembled monolayer on the Au surface by controlling the chemical reaction of Au and disulfide with the Au oxide pattern. It is expected that this easy formation process of the Au oxide layer under the atmospheric condition contributes to the advances of investigation and application of the unique properties of Au oxide.

Indium Tin Oxide Nanowire Arrays as a Saturable Absorber for Mid-Infrared Er:Ca0.8Sr0.2F2 Laser.

We demonstrated a passively Q-switched Er:Ca0.8Sr0.2F2 laser with indium tin oxide nanowire arrays as an optical modulator in the mid-infrared region. In the Q-switched regime, the maximum output power of 58 mW with a slope efficiency of 18.3% was acquired. Meanwhile, the minimum pulse duration and highest repetition rate of the stable pulse trains were 490 ns and 17.09 kHz, corresponding to single pulse energy of 3.4 μJ and peak power of 6.93 W, respectively. To the best of our knowledge it was the first time that indium tin oxide nanowire arrays were employed as a saturable absorber to make pulse lasers carried out at 2.8 μm. The experimental data show that indium tin oxide nanowire arrays can be employed as a competitive candidate for saturable absorber in the field of mid-infrared solid-state lasers.

Efficient lasing in mixtures of helium and fluorine in diffuse discharges formed by runaway electrons

The parameters of stimulated lasing in diffuse discharges formed in mixtures of helium and fluorine in a strongly inhomogeneous electric field are investigated. Lasing is obtained in the visible and VUV spectral regions on the transitions of fluorine atoms and molecules. It is shown that lasing in He – F2 mixtures at a wavelength of 157 nm continues for several half-periods of the discharge current. Due to the homogeneity of the diffuse discharge, the maximum lasing efficiency of the F2 laser is 0.15 %, which corresponds to the efficiency of this type of lasers pumped by pre-ionised transverse volume discharges.

Luminescence Properties of Nd3+‐Doped AlF3‐Based Fluoride Glass in the Vacuum Ultraviolet Region

The development and characterization of the optical properties of a neodymium(III)‐doped 42.75AlF3–33.25CaF2–19BaF2–5YF3 (Nd3+‐doped BCA‐5Y) glass in the vacuum ultraviolet (VUV) region are reported. Photoluminescence peaks at 180 and 227 nm are observed upon excitation with the 157 nm emission of a F2 laser. The dominant 180 nm emission peak is assigned to the 4f25d–4f3 (4I9/2 level) transition in Nd3+. This emission peak exhibits a fast, single exponential luminescence with a 16 ns decay time. The strong luminescence intensity and the fast decay time allude to its prospects for scintillator applications.

A resist-less patterning method of Al thin film on polycarbonate by F2 laser irradiation

In recent years, polycarbonate is gathering interests as light and strong material for smart windows. In this paper, a resist-less patterning method of Al thin film on silicone hard-coated polycarbonate by 157 nm F2 laser irradiation is reported. A photomask placed on Al thin film on polycarbonate and irradiated with F2 laser and the non-irradiated area was removed by KOH aq. The mechanism of the method was examined by XPS measurement and it represents that the surface of Al thin film is modified to Al2O3 by F2 laser irradiation and the Al2O3 surface prevents the chemical etching. The ATR-FTIR and XPS measurements of polycarbonate surface under Al thin film imply that the silicone hard coat layer of polycarbonate was oxidized by F2 laser irradiation even under Al thin film and Al-O-Si bonds might be formed between Al and silicone hard coat interface. The formation of Al2O3 at the surface of Al thin film and Al-O-Si bonds at Al/silicone hard coat interface are expected to give abrasion resistance and strong adhesion for Al thin film on silicone hard-coated polycarbonate. This patterned Al film fabrication method contributes to the manufacturing of electrodes for smart polycarbonate windows.

Luminescence of localized states in oxidized and fluorinated silica glass

Photoluminescence of nominally pure oxidized and fluorinated silica glasses, which have good optical transparency in the optical gap, has been investigated under excitation by excimer lasers. The data are compared with the data for chlorinated silica glass, as well as glass containing OH. The excitation of localized states provides the release and capture of charge carriers, despite the low absorption coefficient. The holes are self-trapped. The recombination in the thermally activated or tunnel process creates luminescence of oxygen-deficient centers. It is concluded that oxidation and fluorination do not change the concentration of localized states, but they change the electronic transitions of the localized states to higher energies. In type III glass with a high OH content, luminescence is suppressed by the presence of hydrogen and appears after irradiation with 7.9 eV photons of the F2 laser. This observation suggests the existence of localized states even in these glasses.

Laser Ablation of Sub-10 nm Silver Nanoparticles

Laser ablation of silver nanoparticles (NPs) was studied with laser post-ionization (LPI) time-of-flight mass spectrometry (TOF MS). Silver NPs containing ∼15 000 Ag atoms (4 nm radius) were deposited by soft landing (energy 3 eV/atom) onto indium tin oxide (ITO)/glass substrates. Laser ablation was performed using frequency-doubled Ti:sapphire nanosecond pulsed laser irradiation at three different wavelengths (371, 401, and 421 nm), whereas for post-ionization, pulses from an F2 laser were used. Laser fluences and time delay dependencies of Ag and In signals were obtained. Using these data, the temperature of the desorption source as well as its time duration were calculated. It was found that the peak temperature of NPs was above their melting point and they cooled down slowly, with temperature decay time of several hundreds of nanoseconds. This anomalous behavior was explained based on a model where the semiconducting ITO substrate is initially transparent to the desorption laser radiation but starts to adsorb it due to the temperature increase arising from heat exchange with NPs. Poor heat conduction in the ITO film creates conditions for long-lived hot spots on the surface and initiates further optical damage of the substrate. No difference in the ablation process due to plasmon resonance was detected, likely due to thermal expansion and melting of NPs during laser irradiation, which then broadens the plasmon absorption band enough to cover all wavelengths used. These results clearly demonstrate that the process of NP interaction with laser radiation is governed not only by initial optical and thermophysical parameters of NPs and the surrounding media, but also by their alteration due to temperature increases during the irradiation process.

Crack suppression of silica glass formed by zoned F2 laser-induced photochemical surface modification of hard silicone thin film coating on polycarbonate

The surface layer of a hard silicone thin film coating on polycarbonate was modified to silica glass (SiO2) through F2-laser-induced photochemical reactions. To obtain samples with higher abrasion resistances, SiO2 films of 1 µm thickness and over were successfully formed without cracking, by zoning the laser-irradiated area of micrometer order. With the conversion of silicone to SiO2, the volumetric shrinkage of the sample was induced, which simply depended on the number of photons, by varying the single-pulse fluence and irradiation time of a F2 laser. The ratio of volumetric shrinkage to the original silicone was estimated to be approximately 0.85, generating tensile stress in SiO2. The stress could be suppressed to be lower than 48 MPa for typical SiO2 by reducing the laser-irradiated area to be of micrometer order. Also, when the length of one side of the irradiated area is 1 mm, the thickness of the SiO2 film is expected to increase to approximately 5 µm.

Crack Suppression of Transparent Protective Layer on Polycarbonate Formed by Fluorine Laser

SUMMARYA transparent, hard silica glass (SiO2) layer was formed on a conventional protective coat made of silicone ([SiO(CH3)2]n) on a polycarbonate plate by the 157 nm fluorine (F2) laser‐induced photochemical modification of silicone into SiO2. Long exposure of the F2 laser to the sample produced cracks on the surface; a metallic mesh mask was set on the sample during F2 laser irradiation to suppress the cracks successfully. As a result, the thickness of the formed SiO2 layer could increase to approximately 1.3 μm without any crack, and the hardness of the sample also increased to approximately 2.8 GPa.

High-precision measurement and standard calibration of triple oxygen isotopic compositions (δ18O, Δ′17O) of sulfate by F2 laser fluorination

A new approach for measuring the triple oxygen isotope composition of sulfate minerals was developed using fluorine gas and an infrared laser to generate O2. A correction for the mass-dependent isotope effect observed during barite fluorination was rigorously calibrated. Analyte gas purification was performed via numerous cryofocus steps and the introduction of an in-line gas chromatograph. As with previous methods, our fluorination technique still requires pairing with independent δ18O measurements made by TC/EA conversion to carbon monoxide, but leads to higher yields than BrF5-based methods. We first calibrated our method against known silicate standards (UWG-2, SCO, NBS-28). Following from this, we report high-precision triple oxygen isotope ratios for international sulfate standards (IAEA-SO-5, IAEA-S O-6, NBS-127), and an internal laboratory standard, JMG. Replicate analyses of JMG yielded a Δ′17O value of −0.057 ± 0.004‰ (standard error), demonstrating per-meg level precision that approaches instrumental limits. We provide δ17O and δ18O values that can be translated into any preferred reference frame for comparison with gases, water, or rocks/minerals. Quantification of differences in triple oxygen isotope composition of the standard reference materials further enables application of this measure and approach on environmental and geological materials.

フッ素レーザにより形成されたポリカーボネート上の透明硬質膜のクラック抑制

A transparent, hard silica glass SiO2 layer was formed on a conventional protective coat made of silicone [SiOCH32]n on a polycarbonate plate by the 157 nm fluorine F2 laser-induced photochemical modification of silicone into SiO2. Long exposure of the F2 laser to the sample produced cracks on the surface; a metallic mesh mask was set on the sample during F2 laser irradiation to suppress the cracks successfully. As a result, the thickness of the formed SiO2 layer could increase to approximately 1.3 µm without any crack, and the hardness of the sample also increased to approximately 2.8 GPa.

Fabrication of hydrophobic and corrosion resistant iron thin film by interference exposure using 157nm F2 laser

A periodic nanostructure of pure iron thin film deposited on silica glass substrate was, on the average of 14 nm in height and at regular intervals of approximately 500 nm, directly fabricated by the interference exposure using a 157 nm F2 laser. The silica glass surface underneath the F2 laser exposed iron selectively swelled to push up the iron thin film. Simultaneously, the surface of the periodically nanostructured iron was photochemically modified into an extremely thin Fe3O4 during the F2 laser exposure. As a result, the samples revealed successfully hydrophobic property and high corrosion resistance to quasi-seawater.

F2-laser-induced micro/nanostructuring and surface modification of iron thin film to realize hydrophobic and corrosion resistant

Nanoswellings of 60 nm height and 500 nm diameter on average of an iron thin film deposited on a silica glass substrate at regular intervals of 2.5 µm were fabricated by the irradiation of a 157 nm F2 laser. The F2 laser was focused on the iron thin film by each microsphere made of silica glass of 2.5 µm diameter, which covered the entire surface of the films. The surface of the silica glass substrate underneath the F2-laser-irradiated iron thin film selectively swelled to push up the film. After the laser-induced micro/nanostructuring, the F2 laser was again irradiated onto the entire surface of the periodic micro/nanostructured iron thin film to form an approximately 2-nm-thick Fe3O4 modified layer. As a result, the samples showed hydrophobicity and high corrosion resistance to 3 wt % NaCl aqueous solution (quasi-seawater). No rust was observed on the samples after the immersion test in the quasi-seawater for 24 h.

Diffusion and Reactions of Photoinduced Interstitial Oxygen Atoms in Amorphous SiO2 Impregnated with 18O-Labeled Interstitial Oxygen Molecules

Both diffusion and reactions of interstitial oxygen atoms (O0) in amorphous SiO2 (a-SiO2) were examined using oxygen-excess a-SiO2 containing 18O-labeled interstitial oxygen molecules (O2) and exposed to F2 laser light (hν = 7.9 eV). Both the F2 laser photolysis of interstitial O2 at 77 K and subsequent heat treatment at ≳200 °C give rise to oxygen exchange between residual interstitial O2 and oxygen atoms in the a-SiO2 network, and these temperatures are far lower than the temperature at which conventional thermal network-interstitial oxygen exchange in unirradiated a-SiO2 occurs (≳700 °C). However, at the initial stage of the low-temperature F2 laser photolysis, an efficient formation of interstitial ozone molecules (quantum yield ≳0.06) via nearly exchange-free diffusion of photogenerated interstitial O0 is observed, and this reaction predominates over the network-interstitial oxygen exchange.

F2-laser-induced surface modification of iron thin films to obtain corrosion resistance

Rustproof, chemical-resistant pure-iron thin films were successfully fabricated by the 157 nm F2-laser-induced surface modification of 50-nm-thick iron thin films. An approximately 2-nm-thick Fe3O4 layer underneath a native Fe2O3 layer of approximately 0.6 nm in thickness was formed on the iron thin films after F2 laser irradiation, as confirmed by X-ray photoelectron spectroscopy. The anodic polarization measurement in a 3 wt % NaCl aqueous solution (quasi-seawater) was conducted; the F2-laser-irradiated samples showed high corrosion resistance to the quasi-seawater. Moreover, no rust was observed on the samples after the immersion test in quasi-seawater for 48 h and longer. The measurement also revealed that the F2-laser-irradiated samples showed high corrosion resistance to a HNO3 aqueous solution. Thus, the micropatterning of iron thin films was demonstrated by the combination of F2 laser irradiation and subsequent HNO3 chemical etching.

Luminescence of unfused 95%SiO2–5%GeO2 amorphous films with fluorine additive: No evidence for presence of GeODC(I) defects found

Photoluminescence (PL) of unfused amorphous germanosilicate films with fluorine additive is studied in 2–8.5eV spectral range. Experiments are based on films deposited on silica substrates by means of the surface-plasma chemical vapor deposition (SPCVD). Films of about 100μm in thickness with “high F” (~4.2wt.%) and “low F” (~0.5wt.%) fluorine content have been fabricated for the experiments. KrF (248nm), ArF (193nm) and F2 (157nm) excimer lasers are used to pump PL. It is found that absorption and luminescence associated with germanium oxygen deficient centers (GeODCs) in “high F” and “low F” films differ. In the “high F” unfused film absorption coefficient of the band at 5eV as well as intensity of the blue PL band at 3.1eV are significantly greater. This film proves features of the so called GeODCs(II), which symbolize twofold coordinated germanium defects in silica network. In the “low F” unfused film absorption band at 5eV is feebly marked. Poorly resolved PL intrinsic to GeODCs(II) can be detected in this film under the KrF laser pump. The most significant PL features are revealed under deeper UV pump by ArF and F2 lasers. Spectral positions of PL bands excited by these lasers correspond to GeODCs(II). However PL decay kinetics dramatically differs from that one intrinsic to GeODCs(II). Noticeable growth of PL intensity caused by permanent (half an hour and more) exposure to ArF and/or F2 lasers takes place, indicating GeODC(II) formation. It is found that considerable body of fluorine additive has the same effect as profusion for “low F” and/or fluorine free germanosilicate amorphous material synthesized by SPCVD. In the “high F” film yield of GeODC(II) PL pumped by the F2 laser remains high. This speaks for the suppression of the competitive 7.6eV absorption band associated with SiODCs(I) by fluorine additive indicating a decrease in the content of this type of defects in the material. High yield of GeODC(II) luminescence pumped by deep UV photons as well as hypothetical similarity of SiODC(I) and GeODC(I) permit one to conclude that GeODCs(II) are the only defects dominating in the materials under study.

Co-Doped ZnS Clusters and Nanostructures Produced by Pulsed Laser Ablation

In this work, we synthesize Co-doped ZnS nanocrystals by pulsed laser ablation and deposition and investigate the composition and dynamics of the plasma produced in the ablation by time-of-flight mass spectrometry. The first experimental observation of Co-substituted ZnS clusters in the ablation plasma is reported; three series of cationic bimetallic clusters Znn–mComSn+, Znn–mComSn+1+, and Znn–mComSn–1+ are identified upon 1064 nm laser ablation of targets of Co (2%) in ZnS. Neutral clusters produced in the ablation are also observed by F2 laser (λ = 157 nm) postionization. Deposits are collected at the same laser ablation wavelength and fluence at which the above bimetallic clusters are observed in the plasma. Analysis by high-resolution transmission electron microscopy and energy-dispersive X-ray spectrometry obtains that the deposits are composed of Zn0.96Co0.04S nanocrystals with local structure showing epitaxial growth between the zinc blende and wurtzite politypes. The deposits show paramagnetic behavior, but no magnetic ordering is observed. The results constitute a first step to help to elucidate the participation of cluster assembly processes in the synthesis of Co-doped ZnS nanomaterials by pulsed laser ablation and deposition.